FIG. 1 is a circuit diagram of a known type of electromechanical switch which can be switched between two alternate states, and which can be left indefinitely in either of the two states. In the context of the present invention an electromechanical switch is a type of electrical switch having mechanical contacts operated as a result of changes in an electromagnetic field flowing in a coil, usually a solenoid.
In FIG. 1 the electromechanical switch is a relay comprising a solenoid RLA and associated ganged contacts SW2 which are biased normally-open. The contacts SW2 can be held closed, against the opening bias, by the associated solenoid coil RLA provided that a sufficient current, known as a holding current, passes through the coil RLA. The combination of the solenoid coil RLA and its associated contacts SW2 may be embodied in an electromechanical relay of known type. The contacts SW2 are located in the live L and neutral N conductors of an AC mains supply to a load LD, so that opening the contacts SW2 will disconnect the load LD from the AC supply.
One side of the solenoid RLA is connected to the live conductor L via diode D1 and resistor R1, and the other side of the solenoid is connected to the neutral conductor N via a manually actuable switch SWφ. A capacitor C1 is connected in parallel with RLA and SWφ. SWφ is a mechanically latchable switch which when manually actuated will close and will remain in the closed state until manually actuated again to change it to the open state where it will remain until actuated again. RLA will be de-energized and energised in tandem with the open and closed states of SWφ, and RLA contacts SW2 will open and close in tandem with the state of RLA and SWφ.
In FIG. 1, when the AC supply is present, capacitor C1 will charge up via diode D1. However the relay coil RLA will not be energized and the associated contacts SW2 will remain open until SWφ is manually closed. When SWφ is closed a current will flow through RLA sufficient to close the load contacts LD and maintain them closed for as long as the switch SWφ remains latched closed. In the absence of any subsequent actuation of SWφ, in the event of loss of and subsequent restoration of the AC supply the load contacts SW2 will always revert to their last state, i.e. to the state prevailing immediately prior to the loss of supply. This is because the switch SWφ is a bistable switch; i.e. its contacts will always remain in their current state until the switch is next actuated.
On the other hand, a monostable switch has just one stable state in that its contacts will normally-be open or normally-closed. For a normally-open device, the switch contacts will be closed when the switch is operated by application of an external force, e.g. manual operation, but they will revert to the open state when the external force is removed. In the absence of an external force, a normally-open switch cannot be used directly to keep a connected load or circuit in a continuously energized state. However such a switch may be used indirectly to achieve this function, as demonstrated in FIG. 2. (In the drawings the same references have been used for the same or equivalent components.)
In FIG. 2, when the AC supply is present, capacitor C1 will charge up via diode D1. However the relay coil RLA will not be energized and its contacts SW2 will remain open until a solid state switch SCR1 is turned on. Switch SW1 is in this case a monostable normally-open type, and when this switch is momentarily held closed SCR1 will turn on and the relay coil RLA will be energized causing its contacts SW2 to close and provide power to the load LD. However, when SW1 is released and opens to its stable state, SCR1 will remain turned on due to the holding current flowing through it from C1 via the relay coil RLA, and the relay will remain energized with its contacts SW2 closed. The coil RLA can be de-energised by momentary closure of a further normally-open monostable switch SW3, which will remove the supply to RLA and also cause SCR1 to turn off. Thus by the use of two normally-open monostable switches SW1 and SW3, relay contacts SW2 can be opened and closed as required.
If the coil RLA were energized and its contacts SW2 closed, and there were a subsequent removal of the AC supply, the contacts SW2 would open due to the removal of current flowing in RLA. However, upon the restoration of the AC supply the contacts SW2 would remain open. It would require the user to operate SW1 to reclose the contacts SW2.
The arrangement of FIG. 1 can therefore be considered to be a circuit with memory in that the relay contacts SW2 will always revert automatically to their previous state on removal of and restoration of the AC supply, whereas the arrangement of FIG. 2 does not have memory in that contacts SW2 will always revert to the open state on removal of and restoration of the AC supply.
Latching (bistable) switches clearly are the preferred choice for applications requiring memory, and these are widely available. However, there are some applications where a latchable switch cannot be used. One example is where a micro switch is placed beneath a plastic membrane. Such membrane type switches are usually operated by applying force via a finger to the membrane directly above the switch. Most standard types of plastic membranes can accommodate only a very limited amount of displacement, e.g. <1 mm, and the switch must change states in response to this limited amount of displacement. In contrast, a latching push type switch will typically require a movement or displacement of >2.5 mm, which is well outside the range of movement achievable with a standard plastic membrane. It would therefore be impractical to use a latching switch under these conditions, and an alternative means must be found to achieve the desired latching switch function with a non-latching switch.
U.S. Pat. No. 7,916,438, which is incorporated herein by reference in its entirety, discloses an electrically latching residual current device comprising a relay whose contacts close automatically on application of a mains supply at or above a certain level. The RCD contacts will open automatically on loss of supply and will reclose automatically on restoration of the supply. The contacts will also open automatically in response to a residual fault current exceeding a certain safe level. In this case the circuit also has means for ensuring that in the event of a loss of supply and restoration of the supply prior to manual resetting of the RCD, the contacts will remain open until intentionally reclosed by the user. This is an important safety feature which prevents the RCD contacts from closing on to a pre-existing fault condition, and is achieved by opening a normally-closed contact in the relay circuit to prevent automatic restoration of the supply current to the relay in the event of the relay being de-energised in response to a fault condition. To this extent, it can be said that this circuit has a built in memory which ensures that the relay reverts to or remains in the state that it was in prior to loss of supply following a fault condition.
However, U.S. Pat. No. 7,916,438 achieves its memory function by the use of an additional electromechanical/electromagnetic means. This presents problems of design complexity, size, cost and user interface in that the resetting mechanism must be accessible to the user. The use of electromechanical or electromagnetic means such as solenoids and relays including permanent magnet relays to achieve the requisite memory function in RCDs and similar products is quite common despite the above mentioned problems because of the absence of an acceptable alternative means to achieve this function.